The present invention relates to a sprung support system, especially for motor vehicles, with pneumatic or hydropneumatic spring elements which are arranged between a sprung mass, e.g., a vehicle body, and a non-sprung mass, e.g., a wheel or axle, and which are constructed in the form of displacement units which possess a first pneumatic chamber whose volume changes according to the respective spring stroke and a second, constant-volume pneumatic chamber which can be connected to and isolated from the first chamber by a valve arrangement controllable as a function of the stroke position of the respective spring element.
A known support system for motor vehicles is shown in DE-B-1,021,732 where a pneumatic concertina arranged between the vehicle body and an axle is connected constantly to a first pneumatic chamber arranged on the vehicle body and is itself coupled, via a line controllable by a rotary slide valve, to a second pneumatic chamber arranged on the body. The rotary slide valve is controlled by a linkage associated between a lever actuating the valve and the associated vehicle axle. As soon as the axle reaches a comparatively highly-compressed or highly-extended position in relation to the vehicle body, the rotary slide valve is closed completely by the linkage, with the result that the second pneumatic chamber is isolated from the first pneumatic chamber and only a correspondingly reduced pneumatic volume between the vehicle body and wheel axle still takes effect. A markedly progressive spring behavior is obtained in this way.
A functionally similar support system is known from DE-A-2,200,356 which achieves an even more progressive spring behavior. Here a pneumatic concertina arranged between a wheel axle and the vehicle body can be isolated from a pneumatic chamber located on the body as soon as the wheel axle moves sufficiently far from a middle position relative to the vehicle body in the compression or the extension direction. At the same time, the axle moving relative to the vehicle body additionally actuates a pneumatic valve, such that, when the wheel axle comes extremely close to the vehicle body, the pneumatic chamber located on the body is subjected to additional pneumatic medium from a pneumatic pressure source, or pneumatic medium is bled from the pneumatic concertina as soon as the wheel axle is moved relative to the vehicle body comparatively far in the extension direction. The pressure rise obtainable thereby in the pneumatic chamber during an extreme compression movement of the wheel axle and the pressure reduction taking place in the pneumatic concertina during an extreme extension movement can have the result that the valve arrangement, which is closed in these operating states, between the pneumatic concertina and pneumatic chamber opens as a result of the pressure difference between the pneumatic chamber and pneumatic concertina. To accomplish this action, the closing members of the valve arrangement are configured as non-return valves.
EP-A-0,166,702 shows a further support system, in which the spring elements are pure pneumatic units, each with two chambers. One of the chambers changes its volume according to the respective spring stroke, while the volume of the other chamber remains constant. A valve arrangement is also located between the chambers so that, with the valve arrangement open, the support system works at a comparatively low spring rate. In other words, in the compression stage of the spring element (the total volume of the two chambers decreases as a result of the spring stroke), the supporting force increases only comparatively slowly; in the extension stage (the total volume of the two chambers increases as a result of the spring stroke), the supporting force decreases only comparatively slowly. This action occurs because the ratio between the change in volume of the two chambers during a spring stroke and the total volume of the two chambers have comparatively low values. In contrast, when the valve arrangement is closed, a comparatively high spring rate is obtained, i.e. the supporting forces increase relatively sharply in the compression stage, while they decrease correspondingly sharply in the extension stage. This action occurs because, during the closing of the connection between the two chambers, only the pneumatic medium of one chamber determines the spring behavior, and the ratio between the change in volume during a spring stroke and the volume of this chamber assumes comparatively high values.
Thus, a changeover between hard and soft springing can be obtained by switching the valve arrangement. This changeover takes place under parameter control for the spring support system according to EP-A-0,166,702, especially as a function of the transverse acceleration of a vehicle. The system of EP-A-0,166,702 also ensures, if appropriate that, in the closing state of the valve arrangement, the two chambers of the pneumatic unit still remain connected via throttle stages or throttle valves and an exchange of pneumatic medium takes place between the chambers as soon as a sufficient pressure difference takes effect. In this way, on one hand, an effective damping of the spring strokes can be achieved; on the other hand, the spring rate of the pneumatic unit changes to a less pronounced extent during the opening and closing of the valve arrangement between the two chambers.
A similar sprung support system for motor vehicles is shown in German Offenlegungsschrift 3,233,160. Once again, pneumatic units, each with two chambers, are provided as spring elements between the vehicle wheels and the vehicle body. One of the chamber changes its volume according to the spring stroke, while the volume of the other chamber remains constant. The valve arrangement between the two chambers and normally assuming its open position is controlled in an axle-related manner, specifically in such a way that a changeover into the closed position takes place when pronounced pressure differences occur between the pneumatic pressures of the pneumatic units assigned to a vehicle axle. In this way, an effect similar to that obtained with conventional transverse stabilizers can be achieved in vehicles, i.e. swaying or rolling movements of the vehicle body are counteracted by a higher spring rate.
Moreover, it is generally known for pneumatic spring units, for example from German Offenlegungsschrift 3,504,217, to assign a chamber of constant volume to the chamber varying its volume during spring strokes and to arrange a randomly actuable changeover valve between these two chambers, so that a changeover between two different spring rates can be carried out.
An object of the present invention is to improve a sprung support system so that undesirable resonant vibrations, especially vibrations of the vehicle body at the so-called body resonant frequency on motor vehicles, are counteracted to a greater extent.
This object has been achieved in accordance with the present invention by providing that the valve arrangement can be changed over between an open position and a closed position at every reversal of stroke direction by a computer-assisted control which determines from the signals of a stroke transmitter arranged between the sprung mass and non-sprung mass not only the respective stroke position, but also the stroke speed and stroke direction.
It is therefore a feature of the present invention that the supporting forces of a spring element can be varied very sharply during a reversal of the direction of movement, in that the valve arrangement can be changed over at the reversal of stroke direction. The result is that the respective vibrations are counteracted to an especially pronounced extent because the change of the supporting forces takes place in phase opposition to the vibratory movements.
For example, if the valve arrangement is closed in the compression stage of the pneumatic spring element before the reversal point of the direction of movement is reached and is opened again at the reversal of the direction of movement (i.e. at the start of the extension stage), the supporting forces increase sharply before the reversal point is reached, while subsequently a pronounced reduction of the supporting force takes place during the changeover of the valve arrangement. This occurs because, in the compression stage with the valve arrangement closed, the pneumatic pressure in the variable-volume chamber of the spring element increases sharply, and a high pressure difference simultaneously occurs between the two chambers. Now, when the valve arrangement is opened at the start of the subsequent extension stage, pneumatic medium flows over from the variable-volume chamber which is under high pressure into the other constant-volume chamber which is under comparatively low pressure. That is, a pronounced reduction in the supporting forces is already taking place in the reversal phase between the opposite directions of movement.
The same applies accordingly, but with an opposite sign, when, in the extension stage of the pneumatic spring element, the valve arrangement is closed before the reversal point between the extension and compression stages is reached and is opened after the reversal of the direction of movement, i.e. at the start of the compression stage. The pneumatic pressure in the variable-volume chamber falls sharply during the final phase of the extension stage because the valve arrangement is closed, and considerable underpressure can occur in relation to the pressure in the constant-volume chamber. When the valve arrangement is opened at the reversal of the direction of movement, however, pneumatic medium flows out of the constant-volume chamber into the variable-volume chamber which is under comparative underpressure, that is to say the supporting forces sharply reduced in the final phase of the extension stage are already markedly increased in the reversal phase between the directions of movement. A movement of the body in relation to the road can be counteracted especially effectively in this manner. In particular, vibrations of the body at the so-called body resonant frequency can be controlled and damped especially effectively.
The relatively high-frequency spring movements of the wheels occurring virtually continuously as the result of road unevenness can, if appropriate, be utilized to vary the height of the vehicle body in relation to the road. For example, if the ground clearance of the vehicle body on one wheel is to be increased, the connection between the variable-volume chamber and the constant-volume chamber of the associated spring unit is kept closed and is opened only when, during a downward movement of the respective wheel relative to the body, an underpressure in relation to the pressure of the constant-volume chamber occurs in the variable-volume chamber. During the subsequent upward stroke of the wheel or during the passage of the reversal point of the wheel movement before the subsequent upward stroke, the connection is then closed again. In this way, the variable-volume chamber can be pumped full of pneumatic medium, until the pressure in the constant-volume chamber has fallen so far that the pressure in the variable-volume chamber, varying as a result of the spring movements, no longer falls below the value of the pressure in the constant-volume chamber in the extension stage of the spring movements.
The present invention also provides for reduction of the ground clearance of the vehicle body in a similar way. In this case, the connection between the constant-volume chamber and the variable-volume chamber is opened during the upward stroke of the wheel in relation to the vehicle body, i.e. in the compression stage, as soon as an overpressure occurs in the variable-volume chamber in relation to the constant-volume chamber. During the subsequent downward stroke of the wheel or the extension stage, the connection is closed. In this manner, the constant-volume chamber can be pumped full of pressure, until the pressure peaks in the variable-volume chamber, occurring in the compression stage of the wheel movements, no longer exceed the pressure in the constant-volume chamber.
A particular advantage of the present invention is that the changeover of the valve arrangement to the vibration-related uncoupling of the sprung mass, the vehicle body where a motor vehicle is concerned, remains very largely unchanged in respect of relatively high-frequency vibrations, such as occur in a motor vehicle, for example, on rough road surfaces. The variation of the spring rate by a changeover of the valve arrangement mainly has an effect on low-frequency vibrations, namely the comparatively slow body vibrations of a motor vehicle.